Electronic trigger circuit



March 11, 1952 M HECHT 2,588,925

ELECTRONIC TRIGGER CIRCUIT Filed May 26, 1950 Patented Mar. 11, 1952` UNITED STATES PATENT` OFFICE Claims.

This invention relates to an improved electronic D. -C. feed back trigger circuit employing a tWostage amplifier and improved circuits which cooperate to provide a highly sensitive and efficient triggering action while operating with a small maximum backlash voltage. y

It is an object of this invention to provide an improved and novel thermionic trigger circuit.

It is a further object of this invention to provide an improved trigger circuit which appreciably reduces back-lash voltage. .It is a further object of this invention to provide an improved thermionic trigger circuit with greater sensitivity and efficiency.

These objects and others auxiliary thereto are obtained by providing a two stage trigger circuit with a common cathode resistance and feeding back a positive voltage from the plate of the second stage to the control grid of the first or input stage.

The novel features of the invention are set forth with particularity in the appended claims. The invention, itself, however, both as to its organization and its method of operation, tovgether with additional objects and advantages thereof, will best be understood from the following description of specic embodiments when read in connection with the accompanying drawings in which:

Figure 1 is a schematic circuit diagram of the trigger circuit.

Figure 2 is a graph in which the measured output current is plotted against the input voltage for the circuit as shown in Figure 1.

Figure 3 is a schematic circuit diagram showing a modification of the circuit shown in Figure 1.

Figure 4 is a graph in which the measured output current is plotted against the input voltage for the circuit shown in Figure 3.

Heretofore the known D. C. feed-back amplifier trigger circuits have left muchrto be desired as regards backlash and consequent efficiency and sensitivity. In the preferred embodiment of the instant invention, these factors are considerx The twin triode is illustrated for the sake of simplicity, it being understood that two triodes or two tubes having multiple grids may be used in place thereof. The negative input D. C. control voltage is applied between terminals I and 2 to the grid 'I and cathode 9 of the rst stage of tube I9, said iirst stage including cathode 9 and anode 5. Cathode 9 of the iirst stage is coupled to cathode I0 of the second stage by a resistance coupling including common resistance I3. As the said input at terminal I becomes more negative the current ow in the iirst stage, from cathode 9 to anode 5, will decrease gradually until a predetermined value of inputvoltage is reached, at which point the current output of the first stage, as measured by meter `3, will drop rapidly for only a minute further increase of the negative input voltage (in a negative direction). At this point, shown by the vertical line at the right of Fig. 2, the trigger action occurs, and the circuit iiips over from one stable set of operating conditions to the other, characteristic of trigger circuits. Continued increase of negative control voltage will cause the circuit to follow to the right on the curve at the lower end of the vertical line to the right of Fig. 2.

If now, the control voltage be given small increments in the positive direction, the current will gradually increase until suddenly it will jump, as shown by the vertical line to the left of Fig. 2, and still further increments of input voltage in a positive direction will cause only a slow further increase in plate current, as shown by the lower line of the top section of Fig. 2.

Just how this action takes place with my circuit will nowbe explained. Assuming that the input voltage is changed from -4 volts to 4.75 volts, the plate current to anode 5 will decrease. This decrease in plate current decreases the IR drop through common cathode resistor I3, increasing, in a positive sense, the voltage between grid 8 and cathode Ill, causing an increase in plate current to anode 6. This causes the potential of anode 6 to move in a negative direction, and this change of potential is fed back, through resistances I4 and I5 to grid '1, still further reducing the plate current to anode 5. The action is cumulative, and as the negative input potential reaches the trigger value, 5.05 volts in Fig. 2, the trigger action occurs.

In a similar manner, the reverse action occurs when the input voltage is changed in a positive direction.

The circuit differs from the well known Schmitt trigger circuit (see O. H. Schmitt, A Thermionic 3 Trigger, Jour. Sci. Insts., 1938, XV, pp. 24-26) both in construction, and in greatly improved performance, as will be explained.

The Schmitt circuit is a two stage D. C. amplifier with positive feedback.V Trigger action takes place when the positive feedback is of such a magnitude that the circuit would not be stable for a given state of current flow in the two stages. If the said Schmitt circuit contained, in the proper connection, a component capable of storing energy, as for instance a capacitive or an inductance or both, then the above condition could lead .to continuous oscillation. Without such an energy storing reservoir, the above circuit can do nothing but slip to a stable state of D. C. current flow whenever the previous state is Yadjusted beyond the limit of stability. An equivalent'oondition would exist in a D. C. amplifier which works on the negative resistance characteristic of certain multigrid tubes, since negative resistance and positive feedback have an equivalent iniluence on the stability of an amplifier.

vThe'brief discussion above is intended to show that positive `feedback or a negative resistance characteristic lare general ways of producing a trigger action. The Schmitt circuit specifies one particular way rof producing'said positive feedback, namely, by means of a common cathode resistor between'two D. C. coupled amplifier stages. Thecoupling from the first to the second stage is accomplished in the Schmitt circuit by means of `a resistor between the plate of the first and the grid ofthe second stage.

The instant -invention has no inters'tage coupling resistor between the plate of the input stage and the grid of the second stage. Instead there isa feedback coupling resistance I4|I5 between the plate of the second stage and the control grid of theinput stage. This resistance I4-l-I5 provides a positive voltage feedback whereas the Schmitt circuit works by positive current feedback because of the common cathode resistor. Figure 1 also shows a common cathode resistor I3 butin this circuit, resistor I3 serves'only to couplerthe input stage to the second stageI be cause there is no other provision in the circuit topprovide for coupling from the rst to the second stage. Y Y

The trigger circuit of this invention 'operates as follows:

(1) Current coupled from input to second-stage by lmeans of a common cathode resistor.

(2) Positive voltage feedback by -means of a resistor between the VAplate .of Athe second stage and the grid of the input stage.

The Schmitt circuit, onfthe other hand, works as follows:

(1) Voltage coupled from the plate of the input stage to the grid of the second stage bymeans of `a resistor.

(2) Positive current feedback by means `of a common cathode resistor. Y

Resistors I4-I-I5 and I6 plus the internal output resistance of the input voltagesource and the amplification of the second stage determine the feedback Voltage ratio.

The same resistors, together with resistor I3, determine simultaneously the correct'bias for the control grid of the first stage, whereas resistors II and I8 together with resistor I3 determine only the correct bias for the control ygrid of the second stage. In order'to have the grids at the correct working point, the cathode resistor must be considerably larger than is recommended for the Schmitt circuit.` The `two stages are, therefore, virtually self-biasing, but the backlash volt- 4 age can be kept ata tolerably small value in spite ofthe high value of resistance I3 (20,000 ohms for instance), whereas the Schmitt circuit has a high backlash voltage for such a large cathode resistor. The reason for this is that the feedback voltage in my circuit can be adjusted independently of the voltage drop across the common cathode resistor, by choosing the proper values for resistors I4, I5 and IB. This provides a feedback suiiicient to overcome a state of equilibrium with a lower backlash voltageV The curvein Figure 2 was plotted jforthe .fol- Y lowing circuit values: Resistance I6=0.4 megohm VResistance I=0.3 megohm Resistance I 2=10,000 ohms Resistance I.'|=3,000 ohms Resistance lli-220,000 ohms Resistance Il=0.3 megohm Resistance I8=0.3 megohm Y Resistance I5=70,000 ohms full value, (adjusted to 41,500 ohms) Plate potential at 4:205 volts Tube I9 is type 6SN'7 Input voltage across 1,2=approx. 5 volt at triggering point Y The backlash voltage was, in this case, approximately 0.07 Volt.

Figure 3 shows a further development of the circuit shown in Figure 1. The resistor VI6 of Figure 1 has been replaced in Figure 3 by the internal plate resistance of the pentode 20. The feedback resistor lll-H5 now serves (together with .resistor I1) as plate load resistor for tube 20. That means that tube 20 can now be used as a pre-amplifier. with the signal voltage fedinto its control grid. This way a considerable decrease in the magnitude of the backlash voltage can be achieved while keeping the .current drain of the source of plate supply voltage practically as low as withouttube 20. Figure 4 shows the measured trigger curve for the circuit values given in 4Figure 3 when:

- Tube I9`is type 6SN7 Tube 20 is type 6SJ7 Resistance I5=70,000 ohms (full value) Resistance I4=0.3 megohm Resistance I2=10,000 ohms Resistance IIL- 1,600 ohms Resistance I I='0.3 megohm Resistance I8'=0.3 megohm Resistance I3=20,000 .ohms

Resistance 2 I =2.0 megohms Plate potential at 4=205 volts Input voltage across I, 2:approx. 3.2 voltsrfol triggering action In V,this case,L the backlash voltage was around 0.02 volt. --Since the pre-amplifier tube 20 is an integral part of the trigger circuit, anyminor drifts previously observable on the grid ofthe input tube of VFigure 1 are considerably reduced at the input oftube 2U because of the amplification between plate and control grid of tube 20.

5 Thus the circuit in Figure 3 supplies not only a smaller backlash voltage but also smaller error sources due to drift than the circuit of Figure l.

The circuit of Figure 3 can provide a smaller backlash voltage not only because of the amplification in tube 20 but also because this tube represents a variable resistance as part of the positive feedback network. The higher said positive feedback is, the larger is the backlash. Now, if the negative voltage input increases, the resistance of tube 20 increases and, consequently, the positive feedback increases. If the negative voltage input decreases, then the resistance of tube 20 decreases and the positive feedback decreases. Since in this circuit the input voltage at which the output current, measured at meter 3, starts to ow, is much less dependent on the amount of positive feedback than that input voltage at which the current stops flowing, which latter occurrence takes place with decreasing negative value of the input voltage, it is clear that this eiect makes an additional contribution to minimizing the backlash voltage.

In the specification, I have explained the principles of my invention and the best mode in which I have contemplated applying those principles, so as to distinguish my invention from other inventions; and I have particularly pointed out and distinctly claimed the part. improvement or combination which I claim as my invention or discovery.

While I have shown and described certain pre- Q put of the rst stage and the input of the second i stage, resistance means connected to the grid and cathode of the second stage for determining the bias potential of said grid, and a positive feedback voltage line connecting the anode of the second stage to the input grid of the rst stage.

2. An electronic trigger circuit comprising a first stage having an input grid, a cathode and an anode. a second stage having an input grid, a cathode and an anode, a common cathode resistor coupling the cathodes of the two stages. and forming the sole means coupling the output of the rst stage to the input of the second stage, resistance means connected to the grid and cathode of the second stage for determining the bias potential of said grid, and resistance means connecting the anode of the second stage to the input grid of the first stage.

3. The circuit claimed in claim 2, in which the said resistance means connecting the anode of the second stage to the input of the rst stage includes the space current path of a multiple grid vacuum tube.

4. The combination claimed in claim 2 in which the said resistance means connecting the anode of the second stage to the input of the first stage includes the space current path of an electron tube having a cathode, an anode, and a control electrode.

5. An electronic trigger circuit characterized by small backlash voltage and consequent great sensitivity, comprising an input stage with an electronic tube, having cathode, anode, and a control electrode, and an output stage with an electronic tube having a cathode, an anode, and a control electrode, current actuated means coupling the output of the input stage to the input of the output stage, and voltage actuated means coupling the output of said output stage to the input of the input stage.

MAYNARD L. HECHT.

REFERENCES CITED UNITED STATES PATENTS Name Date Loughlin Feb. 8, 1949 Number 

